Aliphatic polyesters, such as polyglycolic acid and polylactic acid, are hydrolyzed in organisms, or degraded in the natural environment through microbial metabolism to produce water and carbon dioxide. Aliphatic polyesters have thus attracted attention as biodegradable polymeric materials, which may replace medical materials or commodity resins.
An aliphatic polyester may be obtained via polycondensation of an α-hydroxycarboxylic acid, such as glycolic acid or lactic acid, but this process is difficult to produce a high-molecular-weight polymer. Therefore, a high-molecular-weight aliphatic polyester for use in molding etc. is generally synthesized via ring-opening (co)polymerization of a cyclic ester, such as glycolide, lactide or a lactone.
Specifically, for instance, polyglycolic acid may be synthesized by dehydrating polycondensation of glycolic acid (i.e., α-hydroxyacetic acid) according to the following Equation [I]: However, this polycondensation process using glycolic acid as starting material is difficult to produce high-molecular-weight polyglycolic acid. Therefore, glycolide, whose structure is a bimolecular cyclic ester of glycolic acid (may be referred to as “dimeric cyclic ester” below), is polymerized in a ring-opening mode in the presence of a catalyst, such as tin octanoate, according to the following Equation [II] to synthesize high-molecular-weight polyglycolic acid (or polyglycolide): 
A large scale industrial production of an aliphatic polyester from a cyclic ester, such as glycolide, as starting material essentially requires an efficient and economical supply of a highly pure cyclic ester. However, it has been difficult to synthesize a cyclic ester efficiently and economically. For instance, glycolide is a dimeric cyclic ester whose structure is expressed as one formed by elimination of two molecules of water from two molecules of glycolic acid, but simple esterification of glycolic acid molecules cannot generally produce glycolide, a dimeric cyclic ester but a low-molecular-weight material, such as oligomer. Accordingly, other processes are adopted including, for example, a process where α-hydroxycarboxylic acid oligomer is synthesized and then the oligomer is depolymerized to produce a dimeric cyclic ester.
The conventional technology for production of a dimeric cyclic ester, such as glycolide, is exemplified by the following processes.
U.S. Pat. No. 2,668,162 discloses a process in which a glycolic acid oligomer is ground to powder and the powder is heated at 270-285° C. under an ultra-high vacuum of 12-15 Torr (1.6-2.0 kPa) while feeding the powder to a reaction vessel by extreme bits (about 20 g/hr) to depolymerize it, and resulting vapor containing glycolide produced is collected in a trap. Although this process is feasible on a small scale, it is difficult to enlarge the scale. Therefore, the process is unfit for mass production. In addition, according to this process, the oligomer remains in the reaction vessel as an excessive residue in the form of tar upon the heating, and so the process suffers from low yield and troublesome removal of the residue. Further, in this process, there is a possibility that a glycolide having high melting point may deposit together with byproducts on the inner wall surface of a recovery line to block the line. It is also difficult to recover the accumulated product in the line.
U.S. Pat. No. 4,727,163 discloses a process in which a polyether with a high heat stability is used as a substrate, a block copolymer of the polyether with a small amount of glycolic acid is formed, and the copolymer is then heated and depolymerized to obtain glycolide. However, the block copolymerization process suffers from many steps, complicated operations and high production cost. Further, according to this process, there is a possibility that a glycolide having high melting point may deposit together with byproducts on the inner wall surface of a recovery line to block the line. It is also difficult to recover the accumulated product in the line.
U.S. Pat. Nos. 4,835,293 and 5,023,349 disclose a process in which an α-hydroxycarboxylic acid oligomer is heated into a melt, an inert gas, e.g., nitrogen gas is blown onto the surface of the melt, and a cyclic ester generated and vaporized from the surface of the melt is carried with the gas stream to collect it. According to this process, the rate of formation of the cyclic ester is low, a large volume of the inert gas to be blown requires preheating it and so forth, resulting in a high production cost. In addition, tar formation progresses in the interior of the oligomer melt during the heating, and excessive tar remains as a residue in a reaction vessel. Therefore, this process suffers from low yield and troublesome removal of the residue.
French Patent No. 2,692,263-A1 discloses a process in which an oligomer of α-hydroxycarboxylic acid, its ester or its salt is added to a solvent where a catalyst has been added, and the mixture is stirred under heating to decompose it catalytically. According to this process, the reaction is conducted under normal or elevated pressure using a solvent suitable to carry the cyclic ester in a gaseous state, and the gas phase is then condensed to recover the cyclic ester and the solvent. The publication demonstrates an example where a lactic acid oligomer is used with dodecane as solvent (with boiling point around 214° C.). The present inventors traced the example using a glycolic acid oligomer and dodecane under similar conditions, and it was revealed that tar formation took place as soon as depolymerization started and glycolide formation was stopped at the point when a minute amount of glycolide was formed. What was worse, the reaction residue was so viscous as to require much effort for cleaning it off.
U.S. Pat. No. 5,326,887 and WO92/15572A1 disclose a process in which a glycolic acid oligomer is heated and depolymerized over a fixed bed catalyst to produce glycolide. According to this process, however, a considerable amount of tar is formed upon the heating and remains as a residue. Therefore, the process suffers from low yield and troublesome cleaning on the fixed bed.
In Japanese Patent Laid-Open No. 9-328481 (corresponding to U.S. Pat. No. 5,830,991), the inventors common to the present invention presented a process in which a polar organic solvent with a high boiling point is used in production of a dimeric cyclic ester derived from an α-hydroxycarboxylic acid via depolymerization of an α-hydroxycarboxylic acid oligomer. This process comprises heating a mixture containing the α-hydroxycarboxylic acid oligomer and the high boiling polar organic solvent to a temperature at which depolymerization of the oligomer takes place, in order to form a substantially homogeneous solution phase, further continuing the heating at the temperature to form the dimeric cyclic ester, distilling out the formed ester together with the high boiling polar organic solvent, and recovering the dimeric cyclic ester from the distillate. According to this process, a dimeric cyclic ester can be obtained from an α-hydroxycarboxylic acid oligomer in a high yield while conversion of the oligomer into tar is prevented.
In this document, many polar solvents within the range of 230-450° C. of boiling point are exemplified as high boiling polar organic solvent. Solvents used in the examples are di(2-methoxyethyl) phthalate, diethylene glycol dibenzoate, benzyl butyl phthalate, dibutyl phthalate and tricresyl phosphate, which are all aromatic ester compounds. When the inventors studied depolymerization in more detail using these aromatic ester compounds as high boiling polar organic solvent, prolonged heating of those compounds at the temperature when depolymerization of an α-hydroxycarboxylic acid oligomer takes place was found to tend to thermally deteriorate the aromatic ester compounds. Thermal deterioration of the aromatic ester compounds requires conduction of a purification step, if they are to be used again. In addition, an amount corresponding to deterioration of such an aromatic ester compound must be added again in the reaction of depolymerization. Consequently, it is difficult to reduce further the production cost of a dimeric cyclic ester.
Further, few depolymerization processes using an aliphatic polyester, such as poly(α-hydroxycarboxylic acid) with a high molecular weight, have been described, since conventional processes use mainly α-hydroxycarboxylic acid oligomers as starting material. Japanese Patent Laid-Open No. 12-119269 describes a process in which polyglycolic acid is depolymerized in the solid phase within the temperature range not lower than 200° C. and below 245° C. to produce glycolide. However, this process is not necessarily suitable for efficient mass production of glycolide on an industrial scale. Also, unless heating temperature is controlled rigorously in this process, polyglycolic acid is liable to degenerate into tar.
Mass production of a high-molecular-weight aliphatic polyester, such as poly(α-hydroxycarboxylic acid), will force recycling of product wastes into a major subject. Recycling of mold wastes generated during molding of an aliphatic polyester will be another subject. It will facilitate the recycling to depolymerize a high-molecular-weight aliphatic polyester to the cyclic ester efficiently and economically.